The present invention relates to water treatment, and has as a primary goal the treatment of waste water or of ground water, for environmental remediation or for the production of a higher grade water, for example, to produce water that is useful directly or with blending to produce an industrial, agricultural or potable product water. The invention particularly addresses processes for removal of perchlorate from a water source. The perchlorate may be present in the source either alone or in conjunction with other contaminants or native impurities.
Perchlorate is largely a man-made contaminant, typically found in ground water near munitions depots and firing ranges, rocket launch or test sites, fireworks, munitions and fertilizer factories, rocket fuel manufacturing plants and other military or industrial sites. The material is persistent in the environment, and in many cases has accumulated or has been accumulating in ground water without oversight or regulation for decades. In the United States, high levels of perchlorate have been found at hundreds of sites in more than twenty states, sometimes in a contaminated ground water plume that extends for miles from a site or originating locus of a contaminating activity. Commonly, perchlorate is present as an ammonium, sodium or potassium perchlorate, or a combination thereof; nitrates may also be present, either naturally, or from the same initial contaminating activity or its metabolites; other dissolved or suspended solids characteristic of the underlying ground water may also be present.
In the United States, perchlorate contamination is estimated to threaten or actually impinge upon the drinking water supplies of over fifteen million people in a few western states, and occurrences of less immediate health concern are found in many states from Massachusetts to Texas. Much remains to be elucidated about the effects of perchlorate exposure on human health; uptake and long term risk modeling are not well developed, and a federal maximum contaminant level (MCL) has yet to be set for drinking water, effluent discharge or remediation. Interim standards or state-imposed limits or recommendations for public water supplies have tended to follow the advance of measurement technology and the development of assays with lower detection limits; scientific studies elucidating health effects at these low levels will necessarily lag behind the relatively recent enhancements of measurement capability. Meanwhile, however, some sites or regions with extremely high concentrations in groundwater have been identified, and plumes with clearly unacceptabe high perchlorate concentrations have entered or currently threaten a number of wells in municipal water systems, thus requiring closure of the wells and/or remedial treatment of the groundwater at and/or upstream of the affected well.
Some treatment processes have been instituted since the late 1990s at a number of affected locations, generally operating to achieve perchlorate removal down to the range of 1.5-10 ppb in a (possibly blended) product water. At other sites where there is no threat to drinking water supplies, different limits may be applied when the goal of treatment is to treat and re-inject groundwater to decontaminate a plume, or to supply water of a quality suitable for certain agricultural uses.
At the present time it appears likely that a range of adverse health effects of perchlorate will soon be documented, and that fairly stringent regulatory thresholds will be established for consumption or discharge of this material. Applicant anticipates that, once perchlorate becomes regulated, treatment will be needed for a number of industrial water uses (such as make-up boiler or cooling water) which have little apparent connection with human exposure, but which result in a waste stream having the existing contaminants concentrated many-fold over that of the source water. These applications may require initial perchlorate removal from the raw feed stream, or may later require decontamination of their waste fields, when the source water itself contains even small amounts of perchlorate.
In view of the scale of the problem, it would be highly desirable to identify effective treatment protocols and apparatus for removal of perchlorate.
A number of methods have seen some use.
One technology for remediation of perchlorate in ground water is to pump the water from suitably-positioned wells, pass it through one or more anion exchange resin beds, and either return the treated product to the ground or pass the product to an intended use, while the perchlorate and any other contaminants are captured in the exchange resin, which may then be discarded or regenerated. When the exchange beds are periodically regenerated, the accumulated perchlorate enters the regeneration waste fluids. The spent regeneration fluids may then be disposed of as a solid or otherwise concentrated waste, or may be broken down in a bioreactor or composting process, or may be incinerated on-site. Incineration may be preferred, because this eliminates the possibility of downstream legal liability that might arise if the waste were to be sent elsewhere, and also because, as a practical matter, concentrated perchlorate waste may be highly unstable or explosive. Spent perchlorate-loaded brine from a resin regeneration facility may also be discharged at sea, but it is widely expected that permits to continue such disposition will may not be available in the future.
As a removal tool, ion exchange beds may offer a highly effective treatment, which may, depending on concentration and the other ions that are present, may potentially treat thousands of bed volumes of the feed water between bed regeneration. However, the specific operating characteristics of an ion exchange resin will depend upon the amount and concentrations of components that are present in the water being treated, upon the extent of any pretreatment, and upon the exchange endpoints that are required in view of the intended use and/or the further processes that are to be applied downstream. In the western United States, much of the perchlorate contamination is present in highly mineralized groundwater, for which additional or coordinated treatments would be required to allow perchlorate to be treated efficiently or economically. Typically, different perchlorates—ammonium, sodium and potassium salts—and potentially nitrate, sulfate, and/or other native or contaminant species may be present in a contaminated groundwater plume, and the distribution of these components may influence the capture characteristics, regenerability and overall capacity and aging characteristics of the bed.
The effectiveness of the capture and the regeneration processes and the cost of necessary regeneration chemicals (as well as cost of disposal of spent chemicals) constitute substantial variable factors affecting the commercial feasibility or even the utility of ion exchange treatment. It is therefore desirable to make capture as efficient as possible. Operation should also be stable and predictable. However, western groundwaters may possess relatively high concentrations of sulfate and nitrate, making it difficult to effectively capture perchlorate at the low ppb concentrations required for drinking water standards.
One recent tool developed for such treatment tasks is an anion exchange resin that has been bifunctionalized, for example with two different tertiary amines, to introduce controlled hydrophobicity and limited steric hindrance such that the resin has high selectivity for the less hydrated anions—perchlorate, pertechnate or iodide—over more highly hydrated ions such as nitrate, while still having high overall capture capacity and/or capture rate. One such resin, described in U.S. Pat. No. 6,059,975, has been commercialized as a specialty resin now often cited as suitable for use in site remediation of perchlorate-contaminated plumes. A related multi-step sequential chemical replacement regeneration process has also been developed to optimize use of the bifunctional resin for removal of these ions, as described in U.S. Pat. No. 6,448,299 of the same inventors. These resins are, however, relatively expensive.
Generally, one may expect treatment with any anion exchange resin to effect some degree of perchlorate removal. However, at low concentrations, large residence time in the bed—a ong path length or low flow rate—would seem to be necessary. Moreover, if one runs the feed through a standard anion exchange resin bed, the removal is not selective; nitrate, sulfate and other ions will be taken up to a much greater extent than occurs with the bifunctional specialty resin, reducing bed lifetime and capture efficiency. Any of these ionic species may also later be released or displaced in quantity when other species are presented in the feed, reducing the quality of the product. For example, a sulfate spike in the feed would displace accumulated nitrate from the bed. This possibility is especially to be considered when the concentration of these other co-ions in the ground water is already high. For example, with a groundwater sulfate concentration of 1000-2000 ppm range, the bed will saturate with sulfate, and subsequent operation, while capturing perchlorate, will pass the sulfate; a sulfate spike in the feed will release captured nitrate. More generally, the competitive or multivariate nature of ion-exchange processes does not allow perchlorate removal to be considered in isolation from other aspects of water treatment. When treating water to high purity for potable or industrial use, and one must address the problem of removing all contaminants down to an effective level; when treating solely to remediate perchlorate-laden groundwater on must remove perchlorate efficiently in the presence of other dissolved solids.
There has been relatively little commercial experience with complete or integrated treatment systems, and possible treatment modalities and their operating parameters remain to be explored. To move beyond feasibility studies, or highly subsidized pilots designed to clean up a particular site, it would be desirable to identify further methods or apparatus capable of removing perchlorate, and which may be dependably added to or integrated with a water treatment line of general utility without unexpectedly introducing or impairing the removal of other species.
It would also be desirable to provide an inexpensive yet effective method of removing perchlorate from a source water.
It would further be desirable to provide a well-quantified unit process that may be provided and predictably scaled to dependably pretreat water from a perchlorate-contaminated well, as a front-end treatment such that the product water is treatable by an existing treatment plant or a new plant of conventional design.
It would also be desirable to provide a dependable front end treatment for industrial water that dependably and predictably produces water substantially purer than a specified limit, such that the treated water may be applied to a given use, concentrated and ultimately discharged without exceeding such limit.
It would also be desirable to provide a perchlorate removal treatment that results in a small or limited waste stream or solid waste residue.